This invention is directed to transmissive flat panel electro-optical displays that employ low profile, large area collimated backlights and, in particular, to displays with reflective light sources, such as large, seamless, tiled active matrix liquid crystal displays incorporating fluorescent lamps and/or LEDs in the collimated backlight.
Transmissive flat panel electro-optical displays, such as active matrix liquid crystal displays (AMLCDs) on portable computers, require a backlight. Such flat panel displays include but are not limited to tiled FPDs, monolithic and monolithic-like FPDs. A standard configuration for such a display consists of a thin backlight mounted behind a display panel. The major optical components of the backlight are typically a white rectangular reflective cavity open on the display side, fluorescent type light bulbs that are mounted in the cavity or coupled to the cavity with an optical wave guide, and light conditioning structures such as a diffuser and/or spatially varying neutral density filters that are mounted on the open side of the cavity beneath the display. The light distribution that such backlights produce is approximately Lambertian.
Some types of transmissive flat panel displays require more collimated light than the aforementioned, standard backlight produces. Typically in these situations, additional special purpose light conditioning structures are added to the backlight on top of the diffuser to produce higher gain (i.e., less Lambertian and more collimated light distribution). This is especially true as the span or size of the display increases.
An example of a display requiring a more collimated backlight is the tiled AMLCD described in U.S. Pat. No. 5,661,531, assigned to the present assignee. In this case, the collimated light is necessary to project the image at the pixel plane to a screen disposed a short distance away, with negligible inter-pixel magnification and without intercepting the seams between the tiles. The primary motivation of the present invention is to provide a more optically efficient backlight for such a display.
A second example of applications for a collimated backlight has its motivation in overcoming the well-known fact that liquid crystal type transmissive displays typically have poor contrast with respect to viewing angle. This is because of inherent anisotropy of the liquid crystal (LC) material itself. An alternative to the quasi-Lambertian backlight, directed through an LC display panel with its inherently poor contrast at increasing non-normal view angles, is a more collimated backlight projected through the LC display panel onto a view screen directly on top of the display panel. The collimated light through the LC has a higher average contrast than does the quasi-Lambertian light. The screen redistributes this higher contrast image to viewers at various view angles.
Another example of applications for a collimated backlight is the case where the display manufacturer selects such a backlight for aesthetic, functional, and/or efficiency reasons. In such a configuration, the display is relatively brighter and has better contrast at near normal incidence from which it is normally viewed. Such applications can include displays for aircraft, military, automotive, ATM machine displays, and portable computer displays.
The design and fabrication of backlight assemblies with special light collimating structures for such displays is a uniquely challenging task. An ideal light conditioning structure for the described applications would be inexpensive to fabricate, thin, light weight, and approximately the same length and width as the backlight underneath it. Simple optical structures on one or both sides that produce a large area, uniform, substantially collimated output beam from a near Lambertian backlight source, in close proximity to the light collimating structure. The ideal backlight that uses this light conditioning structure would use efficient, inexpensive, semi-standard, white, diffuse fluorescent lamps or, alternatively, LEDs having output power scaleable both with the number of lamps, and to large sizes. The resultant backlight assembly would have output beam power very nearly equal to the backlight source beam power (high efficiency). Less ideal collimating backlights compromise the degree of collimation, optical power efficiency and fabrication cost and use more specialized lamps. The optical design of such backlight systems may be approached from the two complementary disciplines of imaging and non-imaging optics. Both have relevance to collimating backlights with their special light collimating structures.
In the field of imaging optics it is well known that a small object source placed at the focal distance from a simple thin convex lens produces a collimated beam. However, for the backlight cavity to be the object, the lens sizes and distances for such a configuration are not practicable. Other approaches are required. A lens that more efficiently captures the optical energy from a diffuse source tends to be larger than one that is less efficient. Lenses can be designed and fabricated that do an acceptable job of focusing an image from a source that is approximately the same size or slightly larger than the lens, but at the expense of optical power efficiency. Arrays of micro-lenses do not solve these problems.
In the field of non-imaging optics, the optical designer uses non-image preserving optical principles such as total internal reflection (TIR), multiple reflection and refraction, and light re-circulation between components to produce optical functions that steer and reshape source illumination, rather than strictly image an object or source. Difficult choices between optical performance, efficiency, and cost, are invariably made.
An example of a collimating backlight assembly based on non-imaging optics is the Allied signal SpectraVue(trademark) collimation sheet, described in U.S. Pat. No. 5,739,931. The collimation sheet has thin, long lamps efficiently coupled to one or two parallel edges of a thin planar waveguide by sealed reflectors. A Fresnel lens array is laminated to the top surface of the waveguide. The waveguide is approximately the same length and width as the display to be illuminated. Light rays with appropriate angles are collimated by the Fresnel lens array; those without are totally internally reflected until they intercept the light bulbs and/or reflectors, where they reenter the waveguide for another pass. This configuration produces a very efficient area collimated backlight, but does not apply to either large or multiple diffuse sources, nor does it scale well as the area of the collimation sheet increases (i.e., the input power increases linearly with the size of one edge) but the power required increases as the square of one edge, for constant output power.
Another example of non-imaging optics, special light collimating structure for application in a collimated backlight assembly is brightness enhancement film (BEF) available from 3M Optical Systems of St. Paul, Minn. These light collimating structures are added to a conventional backlight on top of a diffuser to produce higher gain. They are transparent plastic films with a linear array of prismatic grooves in the display side surface. The display side has a specular surface and the backlight side surface is either completely specular or somewhat matte. The collimation action of these filters is based on refraction from the air-groove interface of the small percentage of incident light rays that are within certain polar and azimuth angles. About 50% of the incoming rays are redirected back towards the backlight by a combination of specular mechanisms, including double total internal reflection and multiple reflection and refraction. BEF films are thin, light weight, and relatively inexpensive to mass produce. They can be made in large sizes. However, they also strongly rely on the re-cycling properties of the backlight cavity for system efficiency. Approximately one half of their light output is outside a 30 degree cone with respect to film normal. In applications that strictly require collimated or substantially collimated light, this characteristic is simply not acceptable. The system designer using BEF films in such applications is left with two difficult choices: allow larger angle light through the display with loss of optical performance, or remove large angle light by mechanical apertures or other means, resulting in an inefficient system. The need remains for a light collimating means for such applications that is large, thin, inexpensive, highly collimating, and efficient.
Yet another example of non-imaging optics for a collimated backlight assembly is the teachings of Zimmerman et al, U.S. Pat. No. 5,598,281. This backlight uses an array of apertures to tapered optical elements that use TIR to produce a partial collimation into an array of microlenses for further collimation. These tapered optical elements have planar light input and output surfaces. The ratio of aperture surface area to overall surface area is preferably in the range of twenty to thirty percent.
It is an object of the present invention to provide a large, thin, scaleable, optically efficient, and highly collimated backlight.
The invention is based on the combination of three concepts: the first two concepts produce a novel, strongly collimating, transmissive, light conditioning structure with high back reflectance. The third concept provides for fabrication of an efficient backlight assembly with the inventive light conditioning structure.
The first concept is that a highly collimated light conditioning structure is realized from a modified basic refractive light diffuser, such as described in U.S. Pat. No. 2,378,252. The diffuser, however, is operated in reverse. That is, the entrance and exit surfaces are reversed. This idea also applies to variants of the basic refractive light diffuser, for example as taught in U.S. Pat. No. 5,781,344. The genus of these light conditioning structures is a composite of transparent beads set into an opaque, absorbing, black matrix approximately one bead radius thick. The bead matrix composite is normally on a transparent substrate. The beads either touch or nearly touch the substrate plate. These light diffusers are normally used as projection screens.
A minimally divergent beam from a projector is incident on the hemispherical bead surfaces. The beads refractively focus the projector beam through their close focal point, after which the beam diverges to large view angles. Hence, the view screen function. The black matrix serves as an exit pupil for the beads and also improves the ambient contrast of the light collimating structure screen. In the present modified reversed application, the cavity provides divergent light to the aperture that is near the close focal point of the lens element, whereupon it exits the lens as a more collimated beam. With bead refractive indices or hemispherical lens elements spanning the range of plastics and optical glass, effective bead aperture ratios for quasi-close packed beads are approximately 16%xc2x110%.
The light collimating structure fabricated from the reversed basic refractive diffuser is an excellent collimator, but by itself it is inefficient for two reasons. First, the matrix material is explicitly highly absorbing and of low reflectivity for view screens with good ambient contrast. Much of the light is simply absorbed. The second reason is that the aperture area for light to enter the bead is approximately five percent of the total area. Thus, only a small fraction of the incident light is transmitted through the structure.
The second concept on which the present invention is based addresses the aforementioned first problem by replacing the highly absorbing and low reflectivity matrix of the above refractive light collimating structures with a highly reflective, low absorbing, white material or structure so that light that does not intercept the entrance aperture of the bead will be reflected back into the backlight cavity, rather than be absorbed or otherwise lost.
However, the material characteristics of the opaque, absorbing, black matrix of the basic refractive light collimating structure screen are different from the highly reflective, low absorbing, white matrix of the inventive light collimating structure. In practice, the beaded screen light collimating structure black matrix has the desired properties in thickness of tens of microns, and is quite transparent for thinner sections, whereas the white matrix has the desired properties only in thicknesses of approximately one third of a millimeter to a few millimeters. These material property constraints mean that the radius for the bead or for a hemisphere lens for this invention is greater than or equal to the thickness of the reflective structure on the cavity exit plate. An additional design element is that the sidewalls of the aperture are not highly absorbing. The majority of light rays that intercept the sidewalls are not absorbed, but reflected or scattered.
The third concept on which this invention is based addresses the second aforementioned inefficiency by using the inventive light collimating structure as the cavity exit plate of a substantially light-tight, low absorption, white, high reflectance cavity with included highly reflective light sources. The present invention uses an array of apertures not significantly dependent on TIR or planar input and output surfaces. The preferred aperture ratio is approximately 16%xc2x110%. The inventive backlight assembly includes a substantially light-tight, highly reflective, low absorption cavity containing light sources that are highly reflective. A low loss cavity with included light source s is critical for an efficient, collimated, backlight assembly. Light rays generated from the sources have many opportunities to intercept an entrance pupil of a bead. Those that do are efficiently collimated; those that do not are re-circulated until they do. A backlight consisting of such a cavity and exit plate light collimating structure is not only an excellent collimator, but also a highly efficient collimated backlight.